Sensor apparatus for process measurement

ABSTRACT

A sensor apparatus for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable. The sensor apparatus includes a mounting section configured to be coupled to the vessel, a conductive probe element, a dielectric insert located within the mounting section, a conductive pin coupled to the top end of the probe element, and an electrical connector coupled to the conductive pin. The conductive probe element has a predetermined diameter and is formed to include a section having a reduced diameter adjacent a top end of the probe element and a tapered section providing transition from the probe element to the reduced diameter section. The dielectric insert includes an inwardly tapered section to prevent movement of the probe element in a direction toward the mounting section and an outwardly tapered section. The conductive pin has a larger diameter than the diameter of the reduced-diameter section of the probe element to engage the outwardly tapered section of the dielectric insert and to prevent movement of the probe element in a direction away from the mounting section. The electrical connector is configured to couple the signal line to the probe element through the conductive pin.

This is a division of application Ser. No. 09,008,277, filed Jan. 16,1998, which is a division of application Ser. No. 08/735,736, filed Oct.23, 1996, now U.S. Pat. No. 5,827,985, which is a continuation-in-partof application Ser. No. 08/574,818, filed Dec. 19, 1995, now U.S. Pat.No. 5,661,251.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a sensor apparatus for transmittingelectrical pulses from a signal line into and out of a vessel to measurea process variable.

The process and storage industries have long used various types ofequipment to measure process parameters such as level, flow,temperature, etc. A number of different techniques (such as mechanical,capacitance, ultrasonic, hydrostatic, etc.) provide measurementsolutions for many applications. However, many other applications remainfor which no available technology can provide a solution, or whichcannot provide such a solution at a reasonable cost. For manyapplications that could benefit from a level measurement system,currently available level measurement systems are too expensive.

In certain applications, such as high volume petroleum storage, thevalue of the measured materials is high enough to justify high costlevel measurement systems which are required for the extreme accuracyneeded. Such expensive measurement systems can include a servo tankgauging system or a frequency modulated continuous wave radar system.

There are many applications that exist where the need to measure levelof the product is high in order to maintain product quality, conserveresources, improve safety, etc. However, lower cost measurement systemsare needed in order to allow a plant to instrument its measurementsfully.

Further, there are certain process measurement applications that demandother than conventional measurement approaches. For example,applications demanding high temperature and high pressure capabilitiesduring level measurements must typically rely on capacitancemeasurement. However, conventional capacitance measurement systems arevulnerable to errors induced by changing material characteristics.Further, the inherent nature of capacitance measurement techniquesprevents the use of such capacitance level measurement techniques invessels containing more than one fluid layer.

Ultrasonic time-of-flight technology has reduced concerns regardinglevel indications changing as material characteristics change. However,ultrasonic level measurement sensors cannot work under hightemperatures, high pressures, or in vacuums. In addition, suchultrasonic sensors have a low tolerance for acoustic noise.

One technological approach to solving these problems is the use ofguided wave pulses. These pulses are transmitted down a dual probetransmission line into the stored material, and are reflected from probeimpedance changes which correlate with the fluid level. Processelectronics then convert the time-of-flight signals into a meaningfulfluid level reading. Conventional guided wave pulse techniques are veryexpensive due to the nature of equipment needed to produce high-quality,short pulses and to measure the time-of-flight for such short timeevents. Further, such probes are not a simple construction and areexpensive to produce compared to simple capacitance level probes.

Recent developments by the National Laboratory System now make itpossible to generate fast, low power pulses, and time their return withvery inexpensive circuits. See, for example, U.S. Pat. Nos. 5,345,471and 5,361,070. However, this new technology alone will not permitproliferation of level measurement technology into process and storagemeasurement applications. The pulses generated by this new technologyare broadband, and also are not square wave pulses. In addition, thegenerated pulses have a very low power level. Such pulses are at afrequency of 100 Mhz or higher, and have an average power level of about1 nano Watt or lower. These factors present new problems that must beovercome to transmit the pulses down a probe and back and to process andinterpret the returned pulses.

The present invention relates to a sensor apparatus for transmittingthese low power, high frequency pulses down a probe and effecting theirreturn. Currently, no industrially suitable sensor exists which caneconomically function as a transmission line and withstand typicalindustrial process and storage environments, while maintaining vesselintegrity.

The present invention relates to a single conductor surface wavetransmission line (Goubau line) adapted as a sensor for industrialprocess variable measurement. The present invention incorporates notonly the transmission line function, but also a reference pulse means, asensing function, a process connection mounting function, a sensorfixing means, and a process sealing means all in a single constructionwhich is compatible with standard industrial mounting requirements suchas flanges or threaded connections. The apparatus of the presentinvention also meets the heavy duty demands of an industrial environmentand is suitable for installation in areas of high temperature, highhumidity, high pressure, high chemical aggressiveness, high staticelectricity, high pull-down forces in granular materials, and highelectromagnetic influence. The sensor apparatus is connectedelectrically to a process measurement system electronics which providesits power and signal processing. The sensor apparatus is specificallydesigned to handle high speed, low power, high frequency broadbandpulses which are delivered by the system electronics.

The sensor apparatus of the present invention is particularly adaptedfor the measurement of material levels in process vessels and storagevessels, but is not thereto limited. It is understood that the sensorapparatus may be used for measurement of other process variables such asflow, composition, dielectric constant, moisture content, etc. In thespecification and claims, the term "vessel" refers to pipes, chutes,bins, tanks, reservoirs, or any other storage vessels. Such storagevessels may also include fuel tanks, and a host of automotive orvehicular fluid storage systems or reservoirs for engine oil, hydraulicfluids, brake fluids, wiper fluids, coolant, power steering fluid,transmission fluid, and fuel.

The present invention propagates electromagnetic energy down aninexpensive, single conductor transmission line as an alternative toconventional coax (or otherwise dual) cable transmission lines. TheGoubau line lends itself to applications for a level measurement sensorwhere an economical rod or cable probe (i.e., a one conductor instead ofa twin or dual conductor approach) is desired. The single conductorapproach enables not only taking advantage of new pulse generation anddetection technologies, but also constructing probes in a manner similarto economical capacitance level probes.

As discussed above, the simplest implementations of a singletransmission line in a process measurement probe will not withstand thepreviously discussed rigors of an industrial environment. Further,standard capacitance level probes do not accommodate the transmission ofhigh speed pulses due to the electrical impedance discontinuities whichexist in their assembly.

The present invention solves problems associated with implementing thenew, inexpensive pulse technology by providing an improved mounting,fixing, securing, and sealing sensor apparatus including the combinationof a probe element and transmission line. The present inventionaccomplishes these features while maintaining the electrical operationof a Goubau line including pulse launch, smooth impedance transitionfrom cabling, reference pulse control, transmission through the mountingincluding both transmitted pulse control and reflected pulse control,and facilitation of desired mode propagation.

According to one aspect of the invention, a sensor apparatus is providedfor transmitting electrical pulses from a signal line into a vessel tomeasure a process parameter. The sensor apparatus includes a housinghaving upper and lower mounting sections. The lower mounting section isformed to include a tapered surface adjacent the bottom end of the lowermounting section. The apparatus also includes a conductive probe elementincluding a head having a tapered surface and an elongated conductiveportion extending away from the head. The tapered surface of the head isconfigured to engage the tapered surface of the lower mounting sectionto prevent movement of the probe element in a direction toward themounting section. A dielectric insert is located above the head of theconductive probe element and a conical transitioning pin has a lowerflange configured abut the dielectric insert and a threaded member forcoupling the pin to the probe element to secure the probe element to themounting section.

In the illustrated embodiment, a spring is located between thedielectric insert and a flange of the mounting section to provide anupwardly directed biasing force to the dielectric insert, the pin andthe probe element. The apparatus further includes an electricalconnector coupled to the conical transitioning pin. The connector isconfigured to couple the signal line to the probe element.

According to another aspect of the invention, a sensor apparatus isprovided for transmitting electrical pulses from a signal line into avessel to measure a process parameter. The sensor apparatus includes amounting section configured to be coupled to the vessel and a conductiveprobe element having a predetermined diameter. The probe element isformed to include a section have a reduced diameter adjacent the top endof the probe element. The probe element includes a tapered sectionproviding transition from the probe element to the reduced diametersection.

In the illustrated embodiment a dielectric insert is located within themounting section. The dielectric insert includes an inwardly taperedsection to prevent movement of the probe element in a direction towardthe mounting section and an outwardly tapered section.

Also in the illustrated embodiment, a conductive pin is coupled to thetop end of the probe element. The conductive pin has a larger diameterthan the diameter of the reduced diameter section of the probe elementto engage the outwardly tapered section of the dielectric insert toprevent movement of the probe element in a direction away from themounting section. An electrical connector is coupled to the pin and isconfigured to couple the signal line to the probe element through theconductive pin.

According to a further aspect of the invention, a sensor apparatus isprovided for transmitting electrical pulses from a signal line into andout of a vessel to measure a process variable. The sensor apparatusincludes a mounting section configured to be coupled to the vessel. Themounting section is formed to include a central aperture defined by afirst tapered surface. A dielectric insert has a second tapered surfaceconfigured to engage the first tapered surface of the mounting sectionto prevent movement of the dialectic insert in a direction away from themounting section. The dialectic insert also has a third tapered surface.

In the illustrated embodiment, a conductive transitioning pin has afourth tapered surface configured to engage the third tapered surface ofthe dielectric insert.

Also in the illustrated embodiment, a metallic insert is located abovethe transitioning pin and the dielectric insert. The metallic insert iscoupled to the mounting section to secure the dielectric insert withinthe mounting section. A conductive, flexible or rigid probe element iscoupled to the transitioning pin and extends downwardly through thedielectric insert and into the vessel. An electrical connector iscoupled to the transitioning pin and is configured to couple the signalline to the probe element through the transitioning pin.

In each of the illustrated embodiments at least one of the transitioningpins and the connector includes an aperture and the other of thetransitioning pin and the connector include a pin which slidably engagesthe aperture to permit movement of the probe element relative to themounting section while maintaining the electrical connection.

Additional objects, features, and advantages of the invention willbecome apparent to those skilled in the art upon consideration of thefollowing detailed description of the preferred embodiment exemplifyingthe best mode of carrying out the invention as presently perceived.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the accompanying figuresin which:

FIG. 1 is a diagrammatical view illustrating a single conductor materiallevel sensor for measuring the level of a process variable such as aliquid in a vessel;

FIG. 2 is a sectional view illustrating a first illustrated embodimentof the present invention mounted on a tank flange of a vessel;

FIG. 3 is an exploded sectional view of the apparatus of FIG. 2;

FIG. 4 is a top plan view of a lower process connection flange forconnecting the sensor apparatus of the present invention to the tankflange;

FIG. 5 is a sectional view taken along lines 5--5 of FIG. 4;

FIG. 6 is a sectional view of a single conductor probe element;

FIG. 7 is a top plan view of an upper mounting flange or standoff of thepresent invention;

FIG. 8 is a sectional view taken along lines 8--8 of FIG. 7;

FIG. 9 is a top plan view of a launch plate of the present invention;

FIG. 10 is a sectional view taken through an alternative embodiment ofthe upper mounting flange of the present invention;

FIG. 11 is an exploded sectional view of another illustrated embodimentof the present invention in which the upper and lower mounting flangeshave been modified;

FIG. 12 is a sectional view of another illustrated embodiment of thesensor apparatus of the present invention for mounting a probe elementto a vessel;

FIG. 13 is a sectional view illustrating yet another embodiment of asensor apparatus for mounting a probe element to the vessel; and

FIG. 14 is a sectional view taken through a further embodiment of thepresent invention for mounting a probe element, preferably a cable, rodor wire rope probe, to a vessel.

DETAILED DESCRIPTION OF DRAWINGS

Referring now to the drawings, FIG. 1 is a diagrammatical illustrationof operation of the surface wave transmission line sensor apparatus forprocess measurement. The apparatus 10 is adapted for use with levelmeasurement of a process variable such as a liquid 12 stored within astorage vessel 14.

The present invention includes a mechanical mounting apparatus 16 forsecuring a single conductor transmission line 18 to a top surface 20 ofthe vessel 14. (See vessel flange 34 in FIG. 2.) The mechanical mountingstructure 16 also provides a sealing feature as discussed in detailbelow. The mechanical mounting apparatus 16 enables a transceiver 22 totransmit pulses down the single conductor 18 in the direction of arrow24. Once the pulses reach a top surface 26 of liquid 12, a reflectivepulse 28 is returned back up the conductor 18 in the direction of arrow28.

The transceiver 22 includes processing circuitry which detects thereflected pulses to interpret the return pulses and to generate anoutput signal indicating the level of liquid 12 in the vessel 14.Preferably, the transceiver 22 transmits broadband pulses at a very lowpower level, such as about 1 nW or less average power, or 1 μW or lesspeak power. The frequency of the pulses is preferably about 100 Mhz orgreater.

The present invention is concerned with the mechanical mountingapparatus 16. The improved surface wave transmission line sensorapparatus of the present invention provides several functions for theprocess level measurement. The first function is a mounting function forsecuring the sensor apparatus to the vessel, pipe, chute, bin, or otherprocess environment. A second function of the sensor apparatus of thepresent invention is to provide a seal between an interior region of thevessel and the environment. Yet another function of the sensor apparatusof the present invention is to provide a functional broadbandtransmission line which permits broadband, low power pulses to betransmitted down a single conductor transmission line.

Details of the apparatus of the present invention are illustrated inFIGS. 2 and 3. The entire mechanical assembly is referred to as sensor30. Sensor 30 includes a process connection or lower flange 32 forsecuring the sensor 30 to a mounting flange 34 on the vessel 14. Sensor30 also includes a probe element 36 which is inserted through anaperture 38 formed in lower flange 32 so that a distal portion 40 of theprobe 36 extends into the vessel 14 to provide a single conductor. Lowerflange 32 is coupled to the flange 34 of the vessel by suitablefasteners 42 which extend through apertures 44 formed in lower flange 32and apertures 35 formed in flange 34. It is understood that the lowerflange 32 can have a reduced diameter with a threaded outer portion toprovide a threaded connection to the vessel flange 34, if desired.

Lower flange 32 is best illustrated in FIGS. 4 and 5. Lower flange 32includes four spaced apart threaded apertures 46. A central aperture 38for receiving the probe element 36 is formed to include an outwardlytapered upper surface 48 to improve sealing of and to provide a smoothimpedance transition of the probe element 36 as discussed in detailbelow. Tapered surface 48 is convergent in a direction extendingdownwardly from the top surface 33 of lower flange 32. Preferably,flange 32 is made form stainless steel or other metal material. It isunderstood that flange 32 may be made from another corrosive resistantnonconductive material in accordance with the present invention.

Probe element 36 is best illustrated in FIG. 6. The probe element 36includes a single inner conductor 50 made from stainless steel oranother metal material having an elongated, generally cylindrical bodyportion 52 and a head portion 54 having an increased diameter. Headportion 54 includes a threaded aperture 56 and outwardly taperedsurfaces 58 and 60. Tapered surface 58 is a divergent conical surfaceextending in a direction downwardly from top end 57 of probe. Taperedsurface 60 is a convergent conical surface extending in a directiondownwardly from tapered surface 58.

Preferably, the entire conductor 50 is coated with at least one layer ofTeflon 62 or other insulative material. In the illustrated embodiment, asecond layer of Teflon 64 is added near the head portion 54 of conductor50. It is understood that this area of increased thickness 64 may beexcluded or that the Teflon coating may be single layer having anincreased thickness adjacent head portion 54, or may be an alternatematerial.

The probe element 36 is inserted downwardly through aperture 38 formedin lower flange 32 so that the lower tapered surface 60 of head 54engages the tapered portion 48 of aperture 38. Cooperation of taperedsurface 48 of lower flange 32 with tapered surface 60 of probe element36 provides increased retention force for the probe element 36 toprevent probe element 36 from being forced into the vessel 14 in thedirection of arrow 66 by either external pressure outside the vessel 14,or vacuum or mechanical force inside the vessel 14. Tapered surfaces 48and 60 also provide a seal between the probe 36 and the lower flange 32.

The probe element 36 of the present invention functions as the conductorfor a single line surface wave transmission line. The illustrated probeelement 36 is constructed of a metal rod 50 as discussed above. In otherembodiments, the probe element 36 may be constructed from a cable, wire,wire rope, or any other conductive linear element whether flexible orrigid. The probe element 36 can have a round or other cross-sectionalshape, and can be coated (sheathed) or uncoated. In the presentinvention, such a probe element would include a head similar to head 54with the conductive cable or other conductor extending downwardly fromthe head. The distal end 62 of probe element 36 is important inproviding information related to system calibration, and compensationfactors. In addition, the termination of distal end 62 can affect howwell the sensor functions relative to process materials which contactthe sensor in the direct area where the termination exists. The presentdesign allows for various terminations while preserving the sensorfunction. For example, a ballast weight may be provided on cable-typeprobes. Special ballast design or rod tip constructions can improveinformation from the probe tip. The termination techniques also includethe possibility of mechanically and electrically coupling the distal endof probe element 36 to the vessel 14.

Referring again to FIGS. 2 and 3, an upper flange 68 is positioned overprobe element 36 to secure the probe element 36 to the lower flange 32.Details of the upper flange 68 are illustrated in FIGS. 7 and 8. Upperflange 68 includes a mounting portion 70 formed to include apertures 72.Stainless steel screws 74 extend through apertures 72 and are threadedinto threaded apertures 46 of lower flange 32 to secure the upper flange68 to the lower flange 32. Upper flange 68 includes a top surface 76 anda bottom surface 78. A central aperture 80 extends between the topsurface 76 and bottom surface 78 of upper flange 68. Aperture 80includes an outwardly expanded, tapered surface 82 located adjacentbottom surface 78. Tapered surface 82 is convergent in a directionextending upwardly from bottom surface 78. A first set of threadedapertures 84 is formed in top surface 76 of upper flange 68. A secondset of threaded apertures 86 is also formed in top surface 76 of upperflange 68. The apertures 86 are spaced radially inwardly from the firstset of apertures 84.

The tapered surface 82 of upper flange 68 engages the probe element 36adjacent the tapered surface 58 of head 54. Cooperation of these taperedsurfaces 82 and 58 prevent movement of the probe element 36 in thedirection of arrow 88 due to high pressure inside the vessel 14 and toprovide a seal between the probe 36 and the upper flange. Preferably,upper flange 68 is made from stainless steel or other metal material. Itis understood that flange 68 may also be made from another corrosiveresistant, nonconductive material in accordance with the presentinvention.

The upper and lower flanges 32 and 68 cooperate to rigidly fix or securethe probe element 36 to the process connection such as flange 34 ofvessel 14. Probe element 36 is secured to vessel 14 with sufficientmechanical integrity to withstand high temperature, high humidity, andhigh pressure. Cooperation of tapered surface 48 on lower flange 32 withtapered surface 60 on probe element 36 and cooperation of taperedsurface 82 of upper flange 68 with tapered surface 58 of probe element36 prevent outward movement of the probe element 36 relative to theupper and lower flanges 68 and 32 which might otherwise be caused bypositive process pressures. The cooperation of the tapered surfaces alsoprevents inward or downward movement of the probe element 36 which mightotherwise be caused by negative process pressure, gravity, or processforces such as viscosity, friction, turbulence, mechanical contact withagitators, etc.

The configuration of lower flange 32, probe element 36, and upper flange68 also provide a process seal to minimize or prevent leakage of processmaterials such as gas, liquid, or particulate matter out of thecontainment vessel 14. The improved sealing arrangement of sensorapparatus 30 also prevents or minimizes the ingress of outside elementswhen the vessel is at low pressure. Such sealing is accomplished bysealing materials which fill spaces or gaps between electrical andmechanical elements tightly. For instance, by coating the probe element36 with Teflon layers 62 and 64, an improved seal is obtained betweenthe probe element 36 and upper and lower flanges 68 and 32.

The sensor apparatus 30 provides long term integrity of the seals undera wide range of temperature, pressure, and chemical exposure. Suchsealing is also directed at non-pressurized vessels, where a pressurevector may be created by atmospheric conditions, conditions of fluidhead pressure due to submersion of the mounting, or from processfailures such as failed venting. The sealing surfaces are createdbetween the metallic surfaces of the lower and upper flanges 32 and 68and the metallic surfaces of the probe element 36 by using inertplastics, elastomers, or other dielectric materials such as Tefloncoatings 62 and 64 which are suitable for pressures, temperatures, andchemicals encountered.

Referring again to FIGS. 2 and 3, a stainless steel screw 90 is threadedinto threaded aperture 56 of probe element 36. A high frequencyelectrical connector 92 having a center conductor 94 is coupled to anopening 96 formed in the threaded screw 90 for receiving conductor 94.For example, high frequency connector 92 is an SMA connector.

A stainless steel launch plate 98 includes a central aperture 100 whichis positioned over high frequency connector 92. The configuration oflaunch plate 98 is best illustrated in FIG. 9. Launch plate 98 includesan outer set of apertures 102 and an inner set of apertures 104. Outerapertures 102 are aligned with apertures 84 formed in upper flange 68.Therefore, launch plate 98 is secured by top surface 76 of upper flange68 by suitable fasteners 106. A washer 108 and nut 110 are used tosecure a central portion of launch plate 98 to the high frequencyconnector 92.

In another embodiment of the present invention, the upper flange 68 maybe formed from a plastic or other nonconductive material. In thisinstance, the launch plate 98 is still made from metal. However, thelaunch plate 98 may be coupled to the top surface 76 or the bottomsurface 78 of upper flange 68. In addition, the launch plate 98 may beimbedded within the upper flange 68 in this embodiment.

High frequency connector 92 is then coupled to the transceiver andprocessing electronics by, for example, a first SMA connector 112 whichis coupled to high frequency connector 92. A coaxial cable 114 iscoupled to connector 112 and to a second SMA connector 116 which iscoupled to a processing circuit (not shown) located within an electricalhousing 118. Illustratively, electrical housing 118 includes a bottomlip 120 which fits within a groove 122 formed in launch plate 98.Housing 118 is secured to the sensor apparatus 30 by fasteners 124 whichextend through apertures of the electrical housing 118, throughapertures 104 of launch plate 98, and into threaded apertures 84 inupper flange 68.

The sensor apparatus 30 provides a functional broad band transmissionline. A center conductor of the coax cable 114 is coupled to probeelement 36 through central conductor 94 of high frequency connector 92.The outer shield conductor of coax cable 114 is electrically coupled tolaunch plate 98 and upper flange 68. The configuration of lower flange32, probe element 36, upper flange 68, and launch plate 98 provide acontrolled impedance transition from the micro strip transmission lineand coax cable 114 to the sensor apparatus 30. These elements provide atransition from a two-wire transmission line to a single wire orconductor transmission line such as probe element 36 while maintainingthe mechanical requirements for fixing and sealing the sensor apparatus30.

The transmission line of the present invention functions like a Goubaulaunching mechanism. However, the present launching mechanism isdifferent from the classic Goubau cone. The sensor apparatus 30 providescomplex topologies, as well as material selections, to provide anapparatus which can simultaneously mount and seal the sensor whileproviding a launching mechanism for the pulses.

A controlled impedance transition between the line 114 and the sensor 30provides an initial reflection to provide a reference return pulse tothe system electronics. The sensor 30 creates an inherent compleximpedance mismatch as a transition from the cable 114 to the sensor 30.This impedance mismatch eliminates the need for a "zero time" detectionat the initiating of the pulse itself by creating a reference pulsewhich occurs, in time, significantly later than the initiating pulse. Asthe signal leaves the 50 ohm cable 114, the impedance changes as thesignal enters the sensor apparatus 30. The present invention does notrequire the use of a resistor or other impedance network to provide thereference reflective signal.

The present invention provides a smooth impedance transition after theinitial change so that the pulse continues down the probe element 36.Tapered surface 82 of upper flange 68 and tapered surface 58 of probeelement 36 provide a smooth impedance transition. In addition, taperedsurface 60 of probe element 36 and tapered surface 48 of lower flange 32also provide a smooth impedance transition to reduce the effect of suchimpedances on the transmitted signal. Therefore, the configuration ofthe sensor apparatus 30 provides spacial distribution control so that noabrupt changes in impedance are encountered by the signal. Abruptdiameter changes of the components are minimized in the upper and lowerflanges 68 and 32 or the probe element 36 to minimize impedance changes.

Therefore, the present invention optimizes sensor impedances not just atthe transition between the cable 114 and the sensor 30, but all the waythrough the sensor 30. Impedances through the sensor apparatus 30 arecontrolled by specific geometries. Such optimization is important withlow power, high frequency pulse signals to ensure maximum energytransferred to the probe element 36 by controlling or minimizingundesired reflections of pulsed energy. The sensor 30 incorporatesmechanical features which optimize the total impedance of the sensor toa degree without which the transmission of energy would be severelylimited. These features include, but are not limited to, spacings,material selection, shape, gas filled interstices. Such interstices,which are the size and shape of spacings between the parts, is acritical element. When signals from the process are very good, a versionof the sensor apparatus illustrated in FIGS. 2-9 is sufficient. Forimproved performance, however, cavities or interstices are created inthe flanges 32 and 68 as illustrated in FIGS. 10 and 11. These cavitiesfurther lower the impedance through the mounting.

The transmitted pulse and the reflected pulse characteristics arecontrolled via the impedances determined by the geometries of the launchplate 98, upper flange 68, lower flange 32 and probe element 36. Suchcharacteristics include, but are not limited to, pulse width, amplitude,rise time, fall time, and polarity. Sensor apparatus 30 also minimizes,controls, or eliminates unwanted pulse reflections via impedancesdetermined by these geometries.

As illustrated in FIG. 10, the upper flange 68 may be formed to includean expanded cavity 130 adjacent bottom surface 78. A relatively shorttapered surface 132 provides a transition between aperture 80 andexpanded cavity 130. In the FIG. 10 embodiment, tapered surface 132 isconfigured to engage tapered surface 58 of probe element 36. It isunderstood that lower flange 36 may have a configuration similar to theexpanded cavity of FIG. 10, if desired.

Another embodiment of the present invention is illustrated in FIG. 11.In this embodiment, lower flange 32 includes an expanded cavity 134adjacent aperture 38. Upper flange 68 also includes an expanded cavity136 formed adjacent aperture 80. In certain circumstances, sensorapparatus 30 can be used with the expanded cavities 134 and 136 formedin lower flange 32 and upper flange 68, respectively, without any othercomponents. In other circumstances, the cavities 134 and 136 are filledwith a nonconductive material inserts 138 and 140. Illustratively, theinserts are made from PVC, ceramic, or other nonconductive material. Inthe FIG. 11 embodiment, insert 138 includes a tapered aperture 142configured to abut tapered surface 60 of probe element 36. Insert 140includes a tapered aperture 144 configured to engage tapered surface 58of probe element 36. Inserts 138 and 140 are typically used when highpressure is encountered to prevent movement of the probe element 36 andto provide a larger sealing surface area.

The configuration of sensor apparatus 30 permits the launch plate 98 andthe entire sensor 30 to be positioned outside the vessel 14. Theexternal launching mechanism is protected or shielded fromvessel-external influences by the complex topology of the sensor 30. Themetallic construction of the upper flange 68 and lower flange 32cooperate to minimize electromagnetic influences upon the launchingfunction of the launch plate 98.

The sensor 30 also permits the addition of a resistive component orcomplex impedance network to shunt a static charge away from the systemcircuitry to ground. This is provided by coupling a resistor orimpedance network 146 between screw 90 and high frequency connector 92.The resistor or impedance network can be coupled to washer 108 and toanother washer which is positioned beneath the head of screw 90.Resistor or impedance network 146 provides a discharge bleeder path toshunt static charge away from the system electronics.

The lower seal provided between tapered surface 48 of lower flange 32and tapered surface 60 of probe element 36 which is covered by layers 62and 64 provides a process seal to keep material from entering or exitingfrom vessel 14. If the lower seal should fail, upper seal provided bytapered surface 82 of upper flange 68 and the coated tapered surface 58of probe element 36 prevents the escaping material from entering theelectronics housing 118. As illustrated in FIG. 2, a space is providedbetween the bottom surface 78 of upper flange 68 and a top surface 33 oflower flange 32. This space permits any leakage which passes through thelower seal to dissipate from the side of sensor apparatus 30. Therefore,this gap provides a visual indication of any leakage, and a coolingflamepath, pursuant to the National Electrical Code requirement (Article501).

Another embodiment of the present invention is illustrated in FIG. 12.The sensor apparatus 200 is configured to mount a probe element 202 to avessel 204. Sensor apparatus 200 includes a housing 206 having an uppergland mounting section 208 and a lower gland mounting section 210. Theupper and lower mounting sections 208 and 210 are preferably made fromstainless steel. It is understood that mounting sections 208 and 210 mayalso be made from another corrosive resistant material in accordancewith the present invention. The internal surfaces and dimensions ofhousing 206 are selected to provide desired impedances as previouslydiscussed. Upper mounting section 208 is threadably coupled or welded tolower mounting section 210 by threads or groove 212.

Lower mounting section 210 includes a threaded portion 214 to permit apipe thread connection between sensor apparatus 200 and the vessel wall204. Lower mounting section 210 is formed to include a tapered aperture216 for engaging an upper, outwardly tapered surface 218 of probeelement 202. Preferably, the upper end of probe element 202 includes aninsulation layer 220. Insulation layer 220 is preferably Teflon. Thetapered section 216 of lower mounting section 210 engages the outwardlytapered section 218 covered with insulation 220 to prevent movement ofthe probe element in the direction of arrow 222.

Sensor apparatus 200 further includes a dielectric material insert 224located within housing 206. Preferably, insert 224 is made from plasticor ceramic material. Dielectric insert 224 performs both a mechanicaland electrical function by providing the desired impedance. The desiredimpedance is determined by material selection and shape of the insert224. Housing 206 includes an open air portion 226 located above thedielectric material 224. In an alternative embodiment, region 226 can befilled with another dielectric material such as plastic or ceramic, ifnecessary, to improve the high frequency transmission of signals to theprobe element 202 as discussed below. In either case, the shape anddimension of region 226 is selected to optimize the desired impedance.

A spring 228 is located between an outer flange 230 of dielectricmaterial insert 224 and a bottom flange 232 of lower mounting section210. Therefore, spring 228 applies an upwardly directed biasing force todielectric material insert 224 in the direction of arrow 222.

Probe element 202 is coupled to dielectric material insert 224 by athreaded conical pin 234. Pin 234 is preferably made from stainlesssteel, but other corrosive resistant materials could be used. Pin 234includes a conical body section 236 having an aperture 238 formed in anupper end. The conical body section 236 is shaped to optimize itsimpedance relationship to its surroundings. Pin 234 includes a lowerflange 240 configured to engage a top surface 242 of dielectric insert224. Pin 234 also includes a threaded member 244 configured to becoupled to an upper threaded section 246 of probe element 202 to securethe probe element 202 to the insert 224.

A high frequency connector 248, preferably SMA or BNC type connector islocated within an aperture 250 of upper mounting section 208. Highfrequency connector 248 includes a pin 251 located within aperture 238of conical pin 234. Therefore, the pin 251 is slidable within theaperture 238 to permit some movement of the probe element 202 relativeto the housing 206. For instance, the probe element may move slightlyupwardly in the direction of arrow 222 due to thinning of the insulativelayer 220, upwardly directed pressure, or spring action. The pin 251 isfree-floating within the aperture 238 or alternately within a femaleportion of high frequency connector 248 (SMA or BNC). It will beunderstood that with a female connector 248 an aperture would be formedin connector 248 and a pin would be located on conical pin 234.Therefore, if the probe element 202 moves up or down relative to thehousing 206, the pin 251 slides within the aperture 238 to maintain theelectrical connection between the connector 248 and the probe element202.

In contrast to the embodiment of FIGS. 1-11, only the upward taperedsection 218 of the probe element 202 engages the lower mounting section210. The probe element 202 is held in place by the conical transitioningpin 234. The pin 234 engages the top surface 242 of the dielectricinsert 224, and a top surface 253 of probe element 202 engages a bottomsurface 252 of the dielectric insert 224. When the threaded element 244is tightened within threaded section 246 of probe element 202, thetapered section 218 is moved upwardly in the direction of arrow 222 sothat the insulation 220 above tapered section 218 engages the taperedsection 216 of lower mounting section 210. Spring 228 provides a biasingforce in the direction of arrow 222 to insure that the probe element 202is sealed against the conical surface 216 of lower mounting section 210.For instance, if the insulation layer 220 thins during use, spring 228will move the probe element 202 upwardly to reseal the probe elementagainst the tapered surface 216.

The sensor apparatus 200 permits transmission of high frequency, TDRsignals from the high frequency connector 248 to the probe 202.Impedances of the elements of sensor apparatus 200 are selected tocontrol the rate of change of impedance on the high frequency signal.The spacing between the components of sensor 200 is also selected tominimize abrupt impedance changes which effects signal transmission. Forinstance, the angle of conical section 236 can be changed, if desired.In addition, the dielectric constant of the dielectric insert 224 mayalso be adjusted. A dielectric material may be located within openregion 226 of housing 206.

The sensor apparatus 200 provides process-reliable strain relief for ahigh frequency connection to the TDR probe element 202. The structure ofsensor apparatus 200 enables the probe clement 202 to be mounted in asmall, threaded mounting to the vessel 204. Sensor apparatus 200 furtherprovides a sufficient sealing surface and flame path length forhazardous locations.

Another embodiment of the present invention is illustrated in FIG. 13.The sensor apparatus 300 is provided for coupling a probe element 302 toa vessel 304. The embodiment of FIG. 13 uses an "inverted" taper tosecure the probe element to a housing 306. Housing 306 is formed by anupper gland section 308 which is made from stainless steel. Again othercorrosive resistant materials could be used. Gland section 308 includesupper threaded or grooved apertures 310 and an internal threaded section312. The internal surfaces and dimensions of housing 306 are selected toprovide desired impedances. A metallic insert 314 is threaded into glandsection 308 or alternately may be snapped in and held with a springelement and retaining ring (not shown). Preferably insert 314 isstainless steel. Metallic insert 314 includes an air-filled conicalcavity 316. Cavity 316 may be filled with another dielectric material,if necessary. The conical shape of cavity 316 is selected to providedesired impedances.

A conical steel nut 318 is located within cavity 316. The conical steelnut 318 is shaped to optimize its impedance relationship to itssurroundings. A high frequency electrical connector 320, preferably anSMA or BNC connector, is coupled to a top end of conical nut 318 via anupper decreased diameter pin 319 which is slidable within an aperture321 in connector 320 to permit some movement of probe element 302. Againthe location of the aperture 321 and pin 319 could be reversed. A lowerdielectric insert section 322 is also located within gland section 308.Lower dielectric insert 322 includes an internal aperture 324, and has ashape, dimension and material selected to provide the desired impedance.

Probe element 302 is formed to include a section 326 having a reduceddiameter. A tapered section 328 is formed at one end of reduced-diametersection 326 of probe element 302. Probe element 302 further includes amale threaded member 332 at a top end. An insulative material 334, suchas Teflon, is preferably located on the outside of probe element 302.

During installation, probe element 302 is inserted into the housing 306in the direction of arrow 336. A female threaded section 338 of conicalnut 318 is then threaded onto upper threaded member 332 of probe elementto secure the probe element to the housing 306. The conical nut 318includes a bottom tapered surface 340 configured to force the insulation334 outwardly against tapered surface 342 of dielectric insert 322 toretain the probe element 302 within the housing 306 and prevent movementof the probe element 302 in the direction of arrow 344. Tapered section328 of probe element 302 is configured to engage a tapered section 346of dielectric insert 322 to prevent movement of the probe element 302 inthe direction of arrow 336.

The embodiment of the sensor apparatus 300 provides improved control ofimpedance transition between the various elements of the sensorapparatus 300. Since the bulbous double tapered section of the probeelements shown in the previous embodiments is eliminated in probeelement 302, the shape of probe element 302 facilitates maintaining theratio between the outer diameter of the probe element and the diameterof the mounting elements of sensor apparatus 300. The shape of probeelement 302 therefore helps to minimize impedance changes in the sensorapparatus 300.

The sensor apparatus 300 permits the overall size of the housing 306 tobe reduced while maintaining the desired impedance transitions forpermitting high frequency, TDR signals to be transmitted from the highfrequency connector 320 to the probe element 302.

If necessary, the open area 316 may be filled with a suitable dielectricmaterial to improve impedance transition. The angle of the taper of thedielectric insert 314, or the conical nut 318 may also be changed tocontrol impedance changes or transitions. It is understood that thestatic discharge networks discussed above with reference to FIG. 3 mayalso be used in the sensor apparatus 300 and that other elementsdiscussed in previous figures could be incorporated into apparatus 300such as the spring biasing member discussed previously. A launch plate348 is preferably coupled to a top end of gland section 308 withsuitable fasteners.

Another embodiment of the present invention is illustrated in FIG. 14.The sensor apparatus 400 is provided for coupling a probe element 402 toa vessel 404. Preferably, the probe element 402 is a flexible cable,rod, or wire rope probe element. Sensor apparatus 400 includes an uppermounting gland section 406 having a lower threaded section 408 coupledto a threaded section 410 of vessel 404. The internal surfaces anddimensions of apparatus 400 are selected to provide the desiredimpedances.

Upper gland section 406 consists of an upper internal threaded (orgrooved) portion 412, an inwardly tapered section 414, and a bottomflange 416. A first dielectric insert 418 is located near a bottom endof mounting section 406. Dielectric insert 418 includes an outer flange420 for engaging flange 416 of mounting section 406. An O-ring conductorseal 422 is located adjacent dielectric insert 418. A dielectric wafer424 is located above the first dielectric insert 418, and provides onecontinuous seat for O-ring 422, and insert 418 provides another.

A second dielectric insert 426 is also located within an internal regionof the mounting section 406. Insert 426 may be in two or more pieces ormay be a single piece. In the embodiment shown in FIG. 14, insert 426 isin two pieces. The second dielectric insert 426 is formed to include aconically shaped or tapered outer sidewall 428 for engaging the taperedsection 414 of mounting section 406. Further, the shape, dimension andmaterial of mounting section 406 are selected to provide the desiredimpedances. Therefore, second dielectric insert 426 provides a retentionforce to hold the probe element 402 inside a mounting section 406. Inother words, the outer tapered surface 428 of dielectric insert 426prevents movement of the probe element in the direction of arrow 430. Inthe two piece embodiment of insert 426, an O-ring seal 432 is providedon insert 426 to center and hold the two pieces of the insert 426 duringassembly within the tapered surface 414 of mounting section 406. In theone piece embodiment of insert 426, no O-ring 432 is used.

A second dielectric wafer 434 is located above the second insert 426. Anouter O-ring 436 and an inner O-ring 438 are provided to seal thedielectric wafer 434 against passage of process materials. An electricaltransitioning connector 440 is coupled to a top end 442 of probe element402. Connector 440 is made from stainless steel. Top end 442 isillustratively coupled to connector 440 by a suitable mechanicalconnection such as a threaded coupling, soldering, etc. as indicated atlocation 444. Connector 440 includes an upper tapered or conical section446 and a lower tapered or conical section 448. The conical section 446is shaped to optimize the impedance relationship to the surroundings.Lower conical section 448 is configured to engage a tapered section 450of insert 426 to prevent movement of the connector 440 in the directionof arrow 430. The lower tapered section 448 also engages a taperedsection 452 formed in wafer 434, which serves to facilitate the seal ofO-rings 436 and 438.

A metallic insert 454 is threaded or alternately held with a springelement and retainer (not shown) into top end of mounting section 406.Insert 454 engages flange 455 on housing 406. Preferably insert 454 isstainless steel, but could be made from other corrosive resistantmaterials. The insert 454 includes a conical opening 456 surrounding theconnector 440. It is understood that the open-air space 456 may befilled with another dielectric material, if necessary or desired toimprove electrical transmission of the high frequency, TDR electricalsignals. The shape, dimension and material of the space 456 are selectedto provide the desired impedances.

A high frequency electrical connector 458, preferably an SMA connector,is coupled to an upper end of connector 440 to provide an electricalconnection between a high frequency cable and the probe element 402.Further, the upper end 457 of connector 440 is slidable within anaperture 459 in connector 458 to allow for some movement of probeelement 402. Again an aperture could be formed in the upper end 457 ofconnector 440 and connector 458 could include a pin slidably engagingthe aperture. Insert 454 is installed by way of a tool inserted intothreaded or non-threaded openings 460. The probe element 402 isillustratively coated with an insulative material 462, such as Teflon.Probe 402 can be used without the insulative coating, if desired.

In order to facilitate onsite repair, during installation of the sensorapparatus 400, the top end 442 of probe element 402 can be coupled tothe conical connector 440 prior to installation into the vessel 404.After the mounting section 406 is coupled to the vessel 404, the probeelement 402 can then be inserted downwardly through an aperture 464formed in dielectric insert 426 and into the vessel 404. As discussedabove, engagement of the tapered section 448 of connector 440 with thetapered surface 450 of insert 426 prevents movement of the probe element402 in the director of arrow 430. In addition, engagement of the taperedouter surface 428 of the dielectric insert 426 with the tapered innersidewall 414 of mounting section 404 prevents movement in the directionof arrow 430.

The dielectric constants of insert 418, wafer 424, insert 426, wafer434, and area 456 are selected to minimize impedance transitionsencountered by the electrical signal passing from the connector 458,through the connection pin 440 and into the probe element 402. Theconfiguration of sensor apparatus 400 therefore permits high frequency,TDR signals to be transmitted down the probe element 402 for determiningthe contents in the vessel 404.

Although the invention has been described in detail with reference tocertain preferred embodiments, variations and modifications exist withinthe scope and spirit of the present invention as described and definedin the following claims.

What is claimed is:
 1. A sensor apparatus for transmitting electricalpulses from a signal line into and out of a vessel to measure a processvariable, the sensor apparatus comprising:a mounting section configuredto be coupled to the vessel; a conductive probe element having apredetermined diameter, the probe element being formed to include asection having a reduced diameter adjacent a top end of the probeelement and including a tapered section providing transition from theprobe element to the reduced diameter section; a dielectric insertlocated within the mounting section, the dielectric insert including aninwardly tapered section to prevent movement of the probe element in adirection toward the mounting section and an outwardly tapered section;a conductive pin coupled to the top end of the probe element, theconductive pin having a larger diameter than the diameter of thereduced-diameter section of the probe element to engage the outwardlytapered section of the dielectric insert to prevent movement of theprobe element in a direction away from the mounting section; and anelectrical connector coupled to the pin, the connector being configuredto couple the signal line to the probe element through the conductivepin.
 2. The apparatus of claim 1 wherein at least one of the electricalconnector and the conductive pin includes an aperture and the other ofthe electrical connector and conductive pin slidably engages theaperture to permit movement of the probe element relative to themounting section.
 3. The apparatus of claim 1, wherein the conductivepin includes a conically shaped transition section.
 4. The apparatus ofclaim 3, wherein the conical shaped section has a first diameteradjacent the electrical connector and a second diameter adjacent theprobe element, the second diameter being larger than the first diameter.5. The apparatus of claim 1, further comprising a second dielectricinsert coupled to the mounting section above the first dielectric insertto retain the first